Disc brakes for motorcycles and other vehicles have a function of slowing the rotation of wheels by the friction between discs and pads. The temperature of the discs is greatly increased due to friction heat generated during braking. In recent years, in order to achieve high fuel efficiency to protect the Earth's environment, vehicles have been reduced in weight and the discs have been therefore reduced in thickness. Since a reduction in disc thickness results in a reduction in heat capacity, the temperature of the discs is greatly increased by friction during braking. Therefore, there is a possibility that the discs are rapidly worn away because the discs are tempered and softened.
In view of hardness and corrosion resistance, a low-carbon martensitic stainless steel containing 12% chromium and 0.06% carbon on a mass basis is used to manufacture discs of known disc brakes. The stainless steel is usually machined so as to have a predetermined shape, hardened, and then provided to users. The composition of the stainless steel is designed such that the stainless steel has a hardness of 32 to 38 in HRC (Rockwell C hardness determined according to JIS Z 2245).
After a disc brake made of the martensitic stainless steel is heated to high temperature, particularly 550° C. or more, by friction heat, the hardness thereof is seriously decreased due to the relief of the strain of the disc brake and the precipitation of carbonitrides. Therefore, the hardness of the disc brake can be reduced to less than its lower limit, that is, less than 32 in HRC. In particular, since the discs have been reduced in thickness as described above, the discs need to have high temper softening resistance because the discs are heated to more than 600° C. in some cases.
In order to meet such a need, Japanese Unexamined Patent Application Publication No. 2002-146482 discloses a steel sheet used to improve the disc warpage caused by an increase in temperature. However, the temperature disclosed in this document is up to 600° C. and no technique for improving temper softening resistance, which is a key to prevent the hardness of heated steel sheets from being reduced, is disclosed in the document. Meanwhile, Japanese Unexamined Patent Application Publication No. 2001-220654 discloses another steel sheet with high temper softening resistance. This steel sheet has a hardness of 30 or more in HRC after the steel sheet is maintained at 530° C. or more. However, the hardness of this steel sheet is insufficient because the following steel sheets have been recently demanded: steel sheets that have high temper softening resistance after they are heated to more than 600° C.
It is an object of the present invention to provide a martensitic stainless steel, having high temper softening resistance, for disc brakes. The steel is slightly tempered and softened if the steel is maintained at more than 600° C.; that is, the steel has a hardness of 32 or more or a hardness of 30 or more in HRC after the steel is tempered at 650° C. or 670° C., respectively.